The present invention relates generally to devices for reading coded information which represents characters. The invention is applicable to the reading of any type of character where the reading produces information in binary form which may be interpreted by digital logic circuitry. More particularly, the invention is especially applicable where the information is coded as vertical bars.
One arrangement for analyzing vertical bar coded information is described in French Pat. No. 1,271,150 which was applied for on July 26, 1960 by Compagnie des Machines Bull.
In this particular coding scheme, the bars representing each character are separated by long and short intervals arranged in accordance with a combination code. Specifically, the intervals are defined by the distances separating the leading (or trailing) edges of the bars.
For example, the so-called "two from six" code provides a coding system embracing fifteen combinations covering the figures from 1 to 9 and five special symbols which are used as boundary symbols or beginning- and end-of-word signs. The number of bars selected is seven. This method of coded representation is extremely advantageous in that it tolerates irregularities in the printed characters.
An analysis system such as that described in the above-mentioned French Pat. No. 1,271,150 enables the long and short spaces to be correctly identified so long as the irregularities remain within certain limits. In certain cases however, such as when a number of bars are sufficiently thick to merge, an incorrect number of bars is read. To overcome this disadvantage, the analysis system includes checking means which enable the number of bars read to be checked and, if there is any error, to invalidate the transfer of the coded combination. A buffer store, used in the arrangement for the temporary storage of the six code elements of each character to be identified, receives the signal to invalidate.
In a French Addition No. 79,378 to the main French Pat. No. 1,271,150, the intervals between the bars forming the characters are evaluated by time measuring devices which are adjusted according to the particular constant speed at which the document to be read passes in front of a read head.
These devices comprise at least one timebase and a plurality of amplitude discriminators. An AND gate enables the probability that a character was read to be detected and, by means of a signal from its output, authorizes the transfer of each coded combination to the buffer store.
Reading arrangements such as those described in the above-mentioned patents are particularly advantageous for reading bars printed in magnetizable ink. Reading magnetic characters with known magnetic read heads has advantages as compared with reading performed by means of photosensitive detectors. Detectors of this latter kind, for example photo-diodes, do in fact have the disadvantage of reading not only the normal printed areas but also blemishes. As a result, no distinction is made between the bars, on the one hand, and blemishes such as spots and inking faults (magnetic or otherwise), on the other hand.
Known arrangements for the magnetic reading of characters, such as that described in the above-mentioned French Addition No. 79,378, do include means for discriminating against errors due to inking faults and ferrous inclusions contained in the paper. Specifically, an amplitude discriminator or clipping-threshold amplifier whose threshold level can be adjusted to distinguish printing faults in the magnetized bars may be incorporated along the reading path.
As is described in French Pat. No. 1,295,497, improvements may also be made to character identifying systems by coupling two arrangements containing discriminators together. The discriminators are set for different threshold levels to detect, on the one hand, ferrous inclusions and, on the other hand, inking faults.
Another improvement, such as that described in French Pat. No. 1,375,037, incorporates means for checking the thickness of the bars so as to decrease still further the number of unjustifiably rejected characters by not excluding probable characters in which defective printing has been detected. By moving the document to be read in one direction or the opposite direction, such an arrangement enables a fault at the edge of a bar to be detected by causing the characters to be analyzed in both directions of reading without altering the recognition arrangement.
Despite the various improvements made to recognition systems, in particular for recognizing magnetic characters, the particular approaches described briefly above illustrate the difficulty in providing systems which reliably check the characters read and which do not reject characters when rejection is unwarranted. In effect, various prior art systems do not always allow a character to be recognized as "satisfactory" when, for example, the inking faults and metallic inclusions constitute local blemishes which are negligible in comparison with the mean inking of the bars in the character concerned.
All these considerations show the need to make allowance for all the possible variations in the quality of the printed characters, and in particular those variations which are related to the quality of the medium upon which the characters are printed.
In addition, such prior art reading devices are based on identifying characters as a function of the speed of relative movement between the document to be read and the read head, whether it is the document or the head which moves.
In the particular case of characters formed from vertical bars spaced apart from one another by short and long intervals, with such prior art devices a distinction is made between the two types of interval on the assumption that the predetermined speed of the bars past the front of the read head is constant. If there is any irregularity in the movement of a document to be read, such as might be caused by mis-functioning of the control mechanism, there is a corresponding variation in speed. This speed variation could result in characters being incorrectly read. For example, a long interval might be incorrectly identified as a short interval, or vice versa. Thus, with reading arrangements sensitive to the speed of relative movement between the document to be read and the read head, it is sometimes necessary to provide a more costly mechanism which is sufficiently reliable not to cause variations in speed, which variations could result in confusion between intervals of different kinds.